CN109737946B - Automatic adjustment method for modulation depth in high-precision fiber-optic gyroscope four-state modulation - Google Patents
Automatic adjustment method for modulation depth in high-precision fiber-optic gyroscope four-state modulation Download PDFInfo
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Abstract
The invention discloses an automatic adjusting method for modulation depth in four-state modulation of a high-precision fiber-optic gyroscope, and belongs to the technical field of fiber-optic gyroscopes. On the basis of the existing two closed loops, a third digital closed loop mode is added, when the step wave is reset, sampling values of a front detector and a rear detector are compared, an error signal during 2 pi reset is measured, the size of a modulation signal is adjusted through an FPGA (field programmable gate array), a feedback loop is formed, after the fiber optic gyroscope works for a long time and after the environment, a light source, a circuit and the like change, the 2 pi reset error can still be ensured to be 0, and the stability of the fiber optic gyroscope in long-time work is improved; and the digital closed loop is simple to realize and does not influence other closed loops in the fiber-optic gyroscope.
Description
Technical Field
The invention belongs to the technical field of fiber optic gyroscopes, and relates to an automatic adjusting method for modulation depth in four-state modulation of a high-precision fiber optic gyroscope.
Background
The gyroscope is a core component of an inertial system and is one of important contents of the research of inertial technology. A fiber optic gyroscope is a fiber optic sensor sensitive to angular rate that is actually a ring interferometer based on the Sagnac effect. The high-speed starting device has the advantages of all solid state, low cost, high reliability, high starting speed and the like, is widely applied to the fields of airplanes, submarines, warships, missiles, satellites and the like, and becomes a research hotspot of inertial devices at home and abroad in recent years. The full-digital closed-loop detection scheme of the fiber optic gyroscope is the mainstream detection scheme at the present stage, and the fiber optic gyroscope adopting the type has the characteristics of high precision, large dynamic range, high standard factor linearity and the like. The high theoretical sensitivity of the optical fiber gyroscope has promoted the global research on the optical fiber gyroscope in the last 30 years.
In a fiber optic gyroscope, the Sagnac phase difference Δ φ caused by rotationsProportional to the angular velocity, and the output response of the detector is a cosine function of the phase difference, which causes two problems of the light intensity detected by the fiber-optic gyroscope relative to the phase difference: non-linear and periodic. Therefore, it is a common practice to add a feedback quantity to the fiber optic gyroscope so that a non-reciprocal phase error Δ φ is introduced between two counter-propagating light wavesfCompensating for rotation inducedIs a phase difference ofsThis makes the response of the closed loop fiber optic gyroscope to the rate of rotation substantially linear, which is the fiber optic gyroscope first feedback loop. The phase modulator in the fiber optic gyroscope uses a lithium niobate electro-optical modulator, the voltage applied to the lithium niobate crystal is in a linear relationship with the modulated phase, wherein the voltage value corresponding to the phase of 2 pi is called as the 2 pi voltage of the lithium niobate crystal. The digital phase ramp technique uses digital logic and D/A to realize 2 pi reset by automatic overflow of register. However, due to factors such as temperature drift, the gain of the phase modulation channel is changed, and the 2 pi reset precision is affected. As shown in fig. 1, a 2 pi voltage error can be measured by comparing sampling values of the detector signals before and after 2 pi reset, and when the sampling values of the detector signals before and after 2 pi reset are equal, the 2 pi voltage error is considered to be 0, and when the sampling values of the detector signals before and after 2 pi reset are not equal, the 2 pi voltage has an error. When an error exists, the 2 pi reset is accurately controlled by changing the gain of a step wave phase feedback loop, and the second feedback loop of the fiber optic gyroscope is generally called a second closed loop.
The high-precision optical fiber gyroscope generally adopts four-state modulation, and the phase differences of the four states are respectively-delta phia、-Δφb、Δφb、ΔφaEach state having a duration of tau/2, tau being the transit time of light through the entire length of the fiber coil, and the phase difference satisfying cos delta phia＝cosΔφbI.e. delta phia+Δφb2 pi. The advantage of the four-state modulation over the square-wave modulation is that the four-state modulation can demodulate a 2 pi reset voltage error every two cycles, as shown in fig. 2, where the 2 pi reset error Δ D ═ D1+D3-D2-D4，D1～D4The AD sampling values of four state moments marked with numbers 1-4 in figure 2 are respectively demodulated out through a digital logic circuit, and the square wave modulation detects a 2 pi reset error only when the step wave is reset. When the optical fiber gyroscope works in a low rotating speed state, the step wave reset time is long, so that a long adjustment time is needed for eliminating the 2 pi voltage error, and the four-state modulation can avoid the defects and can quickly workThe voltage of 2 pi is rapidly adjusted.
In a high-precision fiber optic gyroscope, in order to eliminate the influence of back reflection and scattering in a fiber ring on detection precision and reduce the optical kerr effect, a wide-spectrum light source is generally adopted. Light of a single wavelength lambda (intensity of light I)0) The light intensity after interference is I ═ I0[1+cos(2π·ΔL/λ)]It is a simple cosine function, Δ L represents the optical path difference, and a broad-spectrum light source is very different from a monochromatic light source. The wide-spectrum light source includes different wavelength components, and the relationship between the interference pattern of the different wavelength components and the optical path difference is shown in fig. 3. These different wavelength components are added together to cause the interference pattern of the broad spectrum light source to be different from the single wavelength interference pattern, and according to the principle of light interference, two rows of waves, namely, a row of wave a (t) with amplitude a interfere with the wave a (t- τ) delayed by τ (i.e., the optical path difference Δ L ═ c τ, c is the speed of light) in the fiber optic gyroscope, and the intensity of the interference wave is:
wherein the content of the first and second substances,<>denotes the average of time, A*(t) represents the conjugation of A (t).
Venezhinken's theorem states that if the Fourier transform of the amplitude function A (t) is α (f), its autocorrelation function Γ (τ) has a Fourier transform of positive real numbers and is equal to its power spectral density, and thus has
Its inverse transform
f represents frequency.
And defining an autocorrelation function as
Thus, it is possible to provide
The formula of light intensity can be rewritten as
The normalized coherence function of the broad spectrum light source is
The light intensity of the wide-spectrum light source can be obtained
Writable in the spatial domain
Wherein the C (Δ L) function is related to the spectral width of the spectrum,
represents the average frequency of a broad-spectrum light source,
representing the average wavelength of light emitted by a broad spectrum light source. The interference pattern of a broad spectrum light source is shown in fig. 4. This results in the original modulation depth, -delta phib、ΔφaThe corresponding light intensities at the phases are no longer equal.
The second closed loop in the four-state modulation detects the 2 pi voltage error once every two cycles, thereby adjusting the gain coefficient of the step wave. But in a fiber optic gyroscope employing a wide-spectrum light source, the exact modulation depth-delta phi setbAnd delta phiaThe light intensity corresponding to the two states (the phase difference of the two is 2 pi) is not equal any more, and the second closed loop can detect the light intensity corresponding to the two statesAnd (3) demodulating the error of the 2 pi voltage from the A/D sampled digital signal, controlling the gain of the feedback channel after integration, and adjusting the 2 pi voltage to enable the second closed-loop error to be 0, so that the adjusted 2 pi voltage of the second closed-loop is inconsistent with the actually corresponding 2 pi voltage of the lithium niobate crystal. Therefore, when the step wave is reset, the adjusted 2 pi voltage and the actually corresponding 2 pi voltage of the lithium niobate crystal are not accurate to generate errors, as shown in FIG. 5 (delta phi)aSince the second closed loop is adjusted to delta phic) And the performance of the gyroscope is influenced by the error during the step wave resetting.
Under the condition of low rotating speed, the step wave reset time is relatively long, and the influence of the error on the zero offset of the gyroscope is small under the condition of long-time smoothness; however, when the gyroscope is at a certain special rotation speed, the step wave may be frequently reset, as shown in fig. 6, a reset error caused by inaccurate 2 pi voltage during the step wave reset may be detected by the fiber-optic gyroscope, an error generated by the frequent reset at the certain rotation speed is equivalent to introducing an additional rotation speed, and the rotation speed output at this time is equal to the rotation speed equivalent to the actual rotation speed plus the error generated by the frequent reset, which may cause a large change in the scale factor at these rotation speeds, affecting the accuracy of the scale factor.
To suppress this error, it is common practice in the prior art to change the waveform of the four-state modulation, no longer the previous- Δ φa、-Δφb、Δφb、ΔφaThese four states. According to the formula
The previous phase difference is known as- Δ φbAnd delta phiaThe corresponding light intensities are no longer equal but rather the ratio delta phiaSlightly smaller value of delta phinAs shown in fig. 7. Therefore using-delta phin、-Δφb、Δφb、ΔφnThe four new states are used as the modulation depth of the gyroscope, and the method can effectively avoid the reset error caused by adopting a wide-spectrum light source.
But with increasing operating time and environmental changes, anAging of the light source, devices and circuits themselves, such as temperature, vibration, spectral average wavelength, etc. changes in actual applications, which can result in a correlation with-delta phibThe phase difference at which the light intensities are equal is no longer delta phinBut another new phase difference, which causes the inaccurate 2 pi voltage when the step wave is reset. Therefore, in order to solve the problem of stability of the high-precision optical fiber gyroscope in long-time operation, a new method needs to be found, automatic tracking control can be realized, automatic adjustment of modulation depth is guaranteed, and the error is 0 when the step wave is reset.
Disclosure of Invention
The present invention aims to solve the above problems, and provides a solution for automatically adjusting the modulation depth of the four-state modulation by adding a closed-loop feedback on the basis of the original digital closed-loop.
The invention provides an automatic adjustment method of modulation depth in high-precision fiber-optic gyroscope four-state modulation, which adopts a mode of adding a third digital closed loop, compares sampling values of detectors before and after reset when a step wave is reset, measures an error signal when a 2 pi voltage is reset, adjusts the size of the modulation signal through an FPGA (field programmable gate array), and forms a feedback loop, so that automatic tracking, control and adjustment of the modulation depth can be realized, no error is generated when the reset is carried out, and the stability of a scale factor is improved.
Specifically, the automatic adjustment method includes the following steps:
the method comprises the following steps: the fiber optic gyroscope sets an initial modulation depth, the first closed loop works normally, and the second closed loop works normally.
Step two: and detecting a step wave reset signal.
Step three: the value of each tau time is sampled by AD (the voltage value output by the detector is converted into digital quantity), so that the demodulation value DEM _ OLD before the step wave is reset is equal to D31-D32And the demodulation value DEM _ RESET at the time of RESET is D33-D32The DEM _ RESET is subtracted from the DEM _ OLD to obtain the modulation depth error Δ D. Wherein D33AD sampling value for the first tau time after step wave reset, D32AD for the last tau time before step wave resetSampling value, D31Is D32The previous AD sample value is at time τ, which is the time of light travel through the entire length of the fiber coil.
Step four: the FPGA is used for adjusting the size of the modulation signal to form a feedback loop, so that the modulation depth can be automatically adjusted by automatic tracking and control.
In particular, the method comprises the following steps of,
step 4.1: the error Δ D of the modulation depth is integrated.
Step 4.2: and adjusting the gain coefficient C of the modulation depth according to the integrated value.
Step 4.3: will be-delta phiaAnd delta phiaThe digital values of the corresponding registers are multiplied by the gain coefficient C to obtain a new modulation depth-delta phin，ΔφnThe new modulation depth-delta phi of the four-state modulation is obtained at this timen，-Δφb，Δφb，Δφn。
Step five: and returning to the step two, adjusting the modulation depth until the error delta D of the modulation depth is equal to 0.
The invention has the advantages that:
the invention can still ensure that the 2 pi reset error is 0 after the high-precision optical fiber gyroscope works for a long time and the environment, the light source, the circuit and the like change per se, thereby improving the stability of the optical fiber gyroscope in long-time work. And the digital closed loop is simple to realize and does not influence other closed loops in the fiber-optic gyroscope.
Drawings
FIG. 1 is a schematic diagram of a second digital closed-loop detection model in a prior art fiber-optic gyroscope.
Fig. 2 is a diagram of a prior art four-state modulation waveform.
FIG. 3 is a schematic diagram of interference signals of different wavelength components in a prior art broad spectrum light source.
FIG. 4 is a diagram of interference signals of a prior art broad spectrum light source.
FIG. 5 is a schematic diagram of the output error of a detector corresponding to the automatic adjustment of a 2 π voltage in the prior art.
FIG. 6 is a schematic diagram of a detector signal when the fiber-optic gyroscope of the prior art is at a special rotation speed.
Fig. 7 is a diagram illustrating the prior art for avoiding errors by changing the modulation depth.
FIG. 8 is a flow chart of three closed-loop feedback processes provided by the present invention.
FIG. 9 shows detector signals corresponding to each closed loop in the FPGA of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The invention provides an automatic adjusting method of modulation depth in four-state modulation of a high-precision fiber-optic gyroscope.
As shown in fig. 8, which depicts three closed loop feedback operations of the present invention. When the high-precision fiber-optic gyroscope (short for gyroscope) has an additional rotating speed, the detector signal is shown as 9(a), and the demodulated value DEM D is obtained by detecting the difference between every two tau11-D12And then the step height of the step wave is adjusted through feedback to eliminate errors, namely the first closed loop, wherein tau is the transmission time of light passing through the whole length of the optical fiber coil, and D11、D12The sampled values are AD sampled in two adjacent tau time (the voltage value output by the detector is converted into digital quantity). When the voltage of 2 pi has an error, the detector signal is shown in FIG. 9(b), D21+D23-D22-D24The amount of gain error demodulated is the 2 pi voltage error, which is removed by adjusting the serial DA (digital to analog converter) to control the gain, which is a second closed loop, where D is21、D22、D23、D24Is a continuous sampling value of AD in each half tau time. When there is an error in the modulation depth, the detector signal samples the value of τ by AD, and the demodulated value DEM _ OLD before the step wave is reset is set to D as shown in fig. 9(c)31-D32And the demodulation value DEM _ RESET at the time of RESET is D33-D32The DEM _ RESET is subtracted from the DEM _ OLD to obtain the modulation depth error Δ D. While modulating the depthIs composed of four digital registers respectively, -delta phia,-Δφb,Δφb,ΔφaCorresponding to the digital values in four digital registers respectively, the logic part will convert-delta phiaAnd delta phiaRespectively multiplying the digital quantity by a gain coefficient C, adjusting the gain coefficient according to the demodulated error quantity delta D to obtain a new modulation depth-delta phin，ΔφnThe new modulation depth-delta phi of the four-state modulation is obtained at this timen，-Δφb，Δφb，ΔφnThe method forms a correction loop of the modulation depth, and effectively controls the modulation depth of the four-state modulation, thereby ensuring the performance of the scale factor of the gyroscope, which is a third closed loop of the fiber-optic gyroscope. The three closed loops are different in detected error and working frequency, the first closed loop detects the rotating speed and the working frequency is 1/2 tau, the second closed loop detects the 2 pi reset error and the working frequency is 1/tau, the third closed loop detects the error of the modulation depth and the working frequency is the frequency of the step wave reset, so that the three closed loops are different in frequency and are not influenced mutually during demodulation. When the three closed loops work simultaneously, the error amount is 0 when the step wave is reset.
Claims (1)
1. The automatic adjustment method of the modulation depth in the four-state modulation of the high-precision fiber-optic gyroscope is characterized by comprising the following steps: on the basis of the original digital closed loop, a third digital closed loop feedback is added to automatically adjust the modulation depth of the four-state modulation;
the automatic adjustment process specifically comprises the following steps:
the method comprises the following steps: the fiber optic gyroscope sets an initial modulation depth, the first closed loop works normally, and the second closed loop works normally;
step two: detecting a step wave reset signal;
step three: the value of each tau time is sampled by AD, so that the demodulation value DEM _ OLD before the step wave is reset is equal to D31-D32And the demodulation value DEM _ RESET at the time of RESET is D33-D32Subtracting DEM _ RESET from DEM _ OLD to obtain an error delta D of the modulation depth; wherein D33For the first after the step wave resetAD sampled values of time τ, D32AD sampling value for the last tau time before step wave reset, D31Is D32The AD sampling value of a previous tau time, wherein tau is the transmission time of light passing through the whole length of the optical fiber coil;
step four: the FPGA is used for adjusting the size of the modulation signal to form a feedback loop, so that the modulation depth can be automatically adjusted through automatic tracking and control; the method comprises the following specific steps;
step 4.1: integrating the error Δ D of the modulation depth;
step 4.2: adjusting a gain coefficient of the modulation depth according to the integral value;
step 4.3: modulation depth-delta phi for modulating four statesaAnd delta phiaThe digital values of the corresponding registers are multiplied by the gain coefficients respectively to obtain new modulation depths-delta phin，ΔφnThe new modulation depth-delta phi of the four-state modulation is obtained at this timen，-Δφb，Δφb，Δφn；-Δφa,-Δφb,Δφb,ΔφaRespectively corresponding to the digital quantity in the four digital registers;
step five: and returning to the step two, adjusting the modulation depth until the error delta D of the modulation depth is equal to 0.
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| CN110906920B (en) * | 2019-12-10 | 2021-05-14 | 西安航天精密机电研究所 | Method for restraining digital closed-loop fiber optic gyroscope reset error |
| CN111220142B (en) * | 2020-02-25 | 2020-12-15 | 北京航空航天大学 | Signal processing method and device of novel ultrahigh-precision optical fiber gyroscope |
| CN111998843B (en) * | 2020-08-10 | 2022-10-21 | 中国科学院光电技术研究所 | Low-noise digital double-closed-loop fiber-optic gyroscope signal modulation and demodulation method |
| CN113390404B (en) * | 2021-08-18 | 2021-11-23 | 浙江航天润博测控技术有限公司 | Closed-loop control method for fiber-optic gyroscope |
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| CN114719837B (en) * | 2022-05-27 | 2022-09-16 | 北京邮电大学 | Six-state modulation and demodulation method and system for digital closed-loop optical fiber gyroscope |
| CN116026304B (en) * | 2023-03-30 | 2023-06-23 | 中国船舶集团有限公司第七〇七研究所 | Optical fiber gyro 2 pi voltage resetting method based on optical path extension |
| CN116045947B (en) * | 2023-03-31 | 2023-05-30 | 中国船舶集团有限公司第七〇七研究所 | Method for demodulating fiber optic gyroscope based on multistage integral circuit and fiber optic gyroscope |
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